This invention relates to wireless networks, to wireless stations of a wireless network, and in particular to power saving in a wireless station of a wireless network such as a wireless local area network (WLAN). It is for example applicable to a wireless station that conforms to any of the OFDM variants of the IEEE 802.11 standard such as IEEE 802.11a and IEEE 802.11g, and to future derivatives thereof.
Use of wireless networks such as wireless local area networks (WLANs) is becoming widespread. A WLAN may be ad hoc, in that any wireless station may communicate directly with any other wireless station, or have an infrastructure in which a wireless station can only communicate with another station via a central station called an access point (AP). The access point is typically coupled to other networks that may be wired or wireless, e.g., to the Internet or to an intranet. Wireless stations of a wireless network thus wirelessly transmit and receive signals that include modulated data over one or more transmission channels, to transmit data from one wireless station to another.
Portable wireless stations are a popular class of wireless stations used in wireless networks. Such wireless stations include cellular phones, laptop computers, wireless digital cameras, battery-backed access points, etc. Portable wireless stations are typically battery-powered and the battery life is limited, e.g., they can only function for a limited time until the battery needs to be re-charged.
Such portable wireless stations increasingly use one of the orthogonal frequency division multiplexing (OFDM) variants of the IEEE 802.11 standard, e.g., IEEE 802.11a, IEEE 802.11g, etc. OFDM modulation breaks up data to be transmitted into several subcarriers, allowing for greater signal reliability and transmission speeds. Unfortunately, multi-carrier frequency approaches can sometimes translate into higher power consumption in a transmitting and/or receiving wireless station. In other words, portable OFDM wireless stations can have a shorter battery-powered lifetime compared to portable non-OFDM wireless stations, e.g., portable wireless stations conforming to the IEEE 802.11b standard.
Therefore there is a need in the art to provide a power saving method and apparatus for a wireless station of a wireless network that uses OFDM.
A common power reduction technique is to simply turn off or put to a lower-power mode—in either case called “put to sleep” herein—those components that are not being used and to then turn on (“wake up”) those components only when they are needed. The current IEEE 802.11 standard and its derivatives provide for a power save mode that allows for one or more components of a wireless station to be put to sleep, e.g., the physical (PHY) unit of a wireless station, for some portion of its operating period. A wireless station in power save mode essentially puts one or more components to sleep and then periodically wakes up those components necessary to transmit and/or receive transmissions. A neighboring wireless station, e.g., another wireless station in an ad-hoc network or an access point (AP) or client station in an infrastructure network, buffers, e.g., stores, any packets destined to the wireless station in power save mode until it transmits a request for such buffered packets.
Unfortunately the current IEEE 802.11 power save mode has a significant drawback, namely that it is “coarse-grained:” when a wireless station is in power save mode, it only wirelessly receives packets in periodic intervals as defined by the standard. Such a power saving mode can affect the latency, e.g., the responsiveness, of received transmissions.
Therefore there is a need in the art to provide a more fine-grained power saving method and apparatus for a wireless station of a wireless network.